Musannaf Ibn Abi Shayba 18:794

Abu Bakr narrated to us, he said: Ubaydah ibn Humayd narrated to us, from Suhayl ibn Abi Salih, from Sa’id ibn al-Musayyib, he said: I asked him about a free man who killed a slave. He said: "He is killed for him." Then I returned to him, and he said: "He is killed for him. By Allah, if the people of Yemen were to gather against him, I would kill them for him."